S.B.C.C. -> Computer Science -> Courses -> COMSC 111 -> Class Notes

Image Formats

Preliminaries

• How do images get into files?
• How does compression work?
• Are image files kept in HTML pages?
• Palettes? Color Depth

GIF

What are GIFs component parts: Blocks and Extensions. Blocks are the most important. The following is a typical file structure:

• Header
• Logical Screen Descriptor Block
• Graphics Blocks
  • Local Color Table
  • Image or Text
• Trailer Blocks

JPEG

Similar block structure to GIF but with emphasis on compression. Process to produce a JPEG file:

1. Transform the image into a suitable color space.
2. (Optional) Downsample each component by averaging together groups of pixels.
3. Group the pixel values for each component into 8x8 blocks.
4. Convert to frequency space and quantize.
5. Package it up.

PNG

Internet Media Type? (image/png)

• Signature (\211 P N G \r \n \032 \n)
• Chuncks
  • Length
  • Chunk Type
  • Chunk Data
  • CRC